Aromatic hydrocarbons are the building blocks for many industrially important products. They are generally produced in a petrochemicals complex which produces several aromatics. When the complex produces both benzene (BZ) and para-xylene (PX), it is often desirable to transalkylate by-product toluene (T) with trimethylbenzenes (TMBZ), which may be obtained from a C9+ aromatics stream (C9+) derived from reformate. This transalkylation helps maximize the yield of xylenes. The xylenes produced can then be converted to valuable PX by further processing.
It is known to carry out aromatics transalkylation processes using zeolites either in the gas phase, or as a mixture of gas and liquid, i.e., as a partial liquid phase process. The type of processing depends on a variety of factors, including feed, catalyst, operating temperature, etc. Unfortunately, the relationship between catalyst structure and the resultant efficacy for aromatics transalkylation, especially for any particular aromatics transalkylation reaction, such as T/TMBZ, is far from predictable. This is evidenced by the plethora of recent studies and patents on the testing and use of various zeolites for aromatics transalkylation.
Various zeolites, having different crystallographic structures, have been disclosed as catalysts for gas phase aromatics transalkylation processes, particularly for the T/TMBZ transalkylation process, also termed the T/C.sub.9+ aromatics transalkylation process. For example, zeolite beta is taught as a catalyst in J. Das et al., "Transalkylation and Disproportionation of Toluene and C9 Aromatics over Zeolite Beta", Cat. Lett., 23 (1994) 161-168. See also I. Wang et al., "Disproportionation of Toluene and of Trimethylbenzene and their Transalkylation over Zeolite Beta", Ind. Eng. Chem. Res., 29, (1990) 2005-2012; and J. Das et al., "Zeolite Beta Catalyzed C.sub.7 and C.sub.9 Aromatics Transformation", Appl. Cat. A, Gen., 116 (1994), 71-79.
The use of SAPO-5 for gas phase T/C.sub.9+ transalkylation is disclosed by V. Hulea et al., "Study of the Transalkylation of Aromatic Hydrocarbons Over SAPO-5 Catalysts", Microporous Materials, 8 (1997), pp. 201-206. Also, E. Dunitriu et al., in "Transalkylation of the Alkylaromatic Hydrocarbons in the Presence of Ultrastable Y Zeolites, Transalkylation of Toluene with Trimethylbenzenes", (Applied Cat. A: General 135 1996!57-81) teaches using ultra stable Y zeolite for gas phase T/C9+ aromatics transalkylation.
Two T/C9+ transalkylation processes based on mordenite are practiced commercially. One process uses added hydrogen, the other does not. The hydrogen based process, known as the Tatoray Process.RTM., was developed about 20 years ago and is licensed by UOP. The process that does not use hydrogen is known as the Xylenes Plus Process.RTM. and is licensed by IFP. It operates using vaporized feed reacting at low pressures in a non-hydrogen atmosphere. A non-noble metal catalyst is used in a moving bed type reactor.
In both processes, acidic mordenite is the catalyst. The Tatoray Process.RTM. operates at gas phase conditions: about 400 psig, 800-900.degree. F., a WHSV of 1-2 hr.sup.-1 and a H.sub.2 /hydrocarbon (HC) mole ratio of between 2 and 4. Under these conditions mordenite is prone to coking and is thus unstable. The Xylenes Plus Process.RTM. also operates at gas phase conditions. It is believed that the operating pressure ranges from 25-100 psig and the temperature ranges from 600-900.degree. F. Feed to these processes is a mixture of toluene and a C9+ aromatics stream, usually a reformate distillation cut which contains propylbenzenes, trimethylbenzenes and methylethylbenzenes.
Over the years, catalyst development has improved the stability as well as the initial activity of mordenite for this process. Dufresne et al., in U.S. Pat. No. 4,723,048 added a metal from the group consisting of Ni, Pd and Group IB metals in combination with a metal selected from the Group IV metals. French Pat. App. 79-30665 discloses the use of a mordenite with a small pore size, i.e., 4.5-5.0 .ANG. , containing nickel, palladium or silver. Another approach is to modify the zeolite, for example as disclosed in U.S. Pat. No. 4,083,886. Here a mordenite, such as H Zeolon mordenite, is treated in refluxing ammonium hydroxide solution containing 5 wt % NH.sub.3 at a pH of at least 9.5 at 90.degree. C. The resulting catalyst is claimed to have improved activity. U.S. Pat. No. 4,264,473 to Tu et al., also teaches a method of catalyst manufacture for mordenite using an ammoniacal treatment. This treated zeolite is a good catalyst for T/C9+ aromatics transalkylation. U.S. Pat. No. 4,341,914 to Berger discloses a process configuration for T/C9+ transalkylation using mordenite. Here indan, a catalyst poison, is removed from fresh C9+ aromatics feed by distillation. U.S. Pat. No. 4,083,886 to Michalko discloses transalkylation using mordenite subjected to aqueous ammoniacal treatment calcined in intimate admixture with a refractory oxide.
Also, Jung-Chung Wu et al., in "Toluene Disproportionation and Transalkylation Reaction Over Mordenite Zeolite Catalysts", Applied Cat. (1983) pp. 283-294, discusses mordenite catalysts containing copper and palladium. U.S. Pat. No. 5,475,180 to Shamshoum describes a toluene disproportionation process. This process uses mordenite impregnated with 1.0-1.5 wt % nickel, and a toluene feed containing 4-12.5 wt % of heavy reformate, which contains xylenes and C9 aromatics. The process increases xylene production.
Aside from beta, Y, SAPO-5 and mordenite, other zeolites have been taught for gas phase T/C9+ aromatics transalkylation. For example, Absil et al., in U.S. Pat. No. 5,030,787 teach the desirability of using zeolites having a constraint index (C.I.) of from 1 to 3 for this reaction. Disclosed zeolites include steamed beta-containing Pt and MCM-22. Absil et al., also teach that adding metals, including Ni, can improve catalyst life for some of these zeolites.
Also, U.S. Pat. No. 5,441,721 to Valyocsik discloses using zeolite MCM-58, to catalyze a wide variety of conversion processes. One specific example includes "transalkylation of aromatics, in gas or liquid phase". Also, U.S. Pat. No. 5,569,805 to Beck et al. discloses the use of materials having the structure of MCM-58 for the catalytic conversion of aromatic compounds. The gas phase transalkylation of BZ with diethylbenzene is disclosed in Examples 27-29. MCM-58 is similar to SSZ-42 as they have the same topology as indicated by their X-ray diffraction patterns.
There are also numerous U.S. patents that disclose the use of zeolites for partial liquid phase (PLP) transalkylation. By PLP, the person skilled in the art means that the reactants and/or transalkylation products are at a pressure and temperature such that they are substantially in the liquid phase. The "partial" in PLP refers to the fact that there are also gaseous by-products, for example, derived from feed dealkylation, present in the reaction zone. Thus, PLP transalkylation requires a combination of pressures (typically between 300-600 psig) and temperatures (generally below 600.degree. F.), such that the feed and transalkylation products are substantially in the liquid phase. The catalyst typically does not contain a hydrogenation/dehydrogenation metal such as Ni or Pt. Furthermore, hydrogen, a gas, is not added in PLP transalkylation.
An example of a commercial aromatics transalkylation processes that uses PLP transalkylation conditions involves the transalkylation of benzene with diisopropylbenzene to make cumene. Patents related to this include U.S. Pat. No. 4,891,458 to Innes et al., which teaches transalkylation using zeolite beta.
Various other zeolites are known for partial liquid phase transalkylation processes; see U.S. Pat. No. 5,149,894 to Zones et al. (SSZ-25), U.S. Pat. No. 5,653,956 to Zones et al. (SSZ-42), and U.S. Pat. No. 5,254,514 to Nakagawa (SSZ-37). SSZ-37 is believed to be related to NU-87, and the use of NU-87 for transalkylation is disclosed in U.S. Pat. No. 5,178,748 to Casci et al. Also, EP 825151 Al discloses the use of dealuminated NU-87 in the transalkylation of toluene with C9 aromatics in the presence of hydrogen.
Other patents or patent applications that disclose PLP transalkylation include those directed to the use of zeolites SSZ-26, SSZ-33, SSZ-35, SSZ-44, and CIT-1. SSZ-26 is described in U.S. Pat. No. 4,910,006; SSZ-33 and Al-SSZ-33 are described in U.S. Pat. No. 4,963,337; SSZ-35 is described in U.S. Pat. No. 5,316,753; SSZ-44 is described in U.S. Pat. No. 5,580,540 and CIT-1 is described in U.S. Pat. No. 5,512,267. These patents are all incorporated herein by reference in their entirety.
The patents covering these zeolites teach their utility for transalkylation of benzene, toluene and xylene using a transalkylating agent which is a polyalkyl aromatic hydrocarbon containing two or more alkyl groups that each may have from two to four carbon atoms. Preferred polyalkyl aromatic hydrocarbons are the dialkylbenzenes, especially diisopropylbenzene. There is no suggestion in any of the patents that these known zeolites would be useful catalysts for gas phase transalkylation in the presence of added hydrogen. Nor is there any suggestion that these zeolites would be especially effective for the gas phase T/TMBZ transalkylation process in the presence of a mild hydrogenation metal.
In the T/TMBZ transalkylation reaction, equilibrium sets the toluene and TMBZ conversion, as well as the yield of product benzene and xylene. An ideal catalyst would give an equilibrium concentration of benzene and xylenes with no light (C.sub.5-) or heavy (C.sub.10+) aromatics.
Depending on the reactants or desired products, some selective dealkylation may be advantageous. For example, when the feed to a T/TMBZ transalkylation process contains aromatic hydrocarbons having ethyl or propyl groups, selective dealkylation of these alkyl groups (without methyl group dealkylation) can result in an improved process. For example in T/TMBZ transalkylation, deethylation of methylethylbenzene to toluene reduces the amount of an undesirable component and produces a useful feed component.
It would be desirable to develop a T/TMBZ transalkylation process where the catalyst is more active and more stable than current commercial catalysts. It would also be advantageous if this improved catalyst has xylene and benzene yields that are close to the equilibrium yields, e.g., at least 90% of the xylene equilibrium yield and less than 120% of the benzene equilibrium yield. These improvements would result in a transalkylation process that operates at lower temperature and pressure, and has higher yields of xylenes, and lower yields of BZ (due to less feed degradation). In addition, the improved catalyst stability would lead to longer cycle lengths, so the process would have lower capital investment, lower operating costs, and higher cash flow. If the catalyst were also able to selectively dealkylate ethylated and/or propylated hydrocarbons, for example, deethylate methylethylbenzene to toluene and/or dealkylate propylbenzene to benzene, this would be especially advantageous.
There is still a need for improved catalysts and processes for gas phase aromatics transalkylation, especially for T/TMBZ transalkylation. Unfortunately, because zeolite catalysts have such a wide variety of chemical structures, one cannot yet predict how useful a particular zeolite will be for any particular transalkylation process. Nor can one predict whether to use gas phase or liquid phase transalkylation conditions. The current state of the art is such that one cannot predict how active or how selective a particular zeolite catalyst, having a unique chemical structure, will be for any particular transalkylation reaction under particular transalkylation conditions. The structure/activity relationship for zeolites in, for example the T/TMBZ transalkylation process, is still being developed.
One object of the present invention is to provide a gas phase T/TMBZ transalkylation process that has a xylene yield that is close to equilibrium. Another object of the invention is to provide an improved gas phase T/TMBZ transalkylation process wherein the catalyst has good stability and selectivity.